Conventionally, as a method of detecting the discharge/non-discharge of ink from an ink-jet head or the discharge state of ink, for example, the ink droplet detector disclosed in Japanese Patent Laid-Open No. 11-170569 is available. This detector has a function of determining the discharge state of ink from the ink-jet head. Upon detecting a nozzle that discharges no ink, the detector notifies the user of the ink-jet printer with an error warning or the like, thereby allowing the user to prevent printing of any faulty image.
The disclosed technique of detecting the discharge/non-discharge of ink however, has the following problems.
(1) An ink droplet is charged, and whether ink is discharged or not is detected by detecting charge (induced charge) when the ink droplet passes. However, the charge given to an ink droplet concentrates on the surface of the ink droplet, and hence the detectivity based on such an ink droplet is low. If the amount of ink discharged is small, in particular, only a slight output can be obtained, posing a problem in terms of reliability.
(2) In order to solve problem (1) described above, the electric field between the ink-jet head and the ink detector may be increased by applying a high voltage of about 100 V between them so as to increase the amount of charge given to an ink droplet. This, however, requires an enormous cost, and a high voltage is generated and applied inside the apparatus, posing a problem in terms of safety.
(3) In addition, since a larger amount of charge given to ink droplets must be collected, charge must be detected from a plurality of ink droplets. This takes more time, and the amount of ink waste increases because a plurality of ink droplets are discharged. In addition, when detection is performed on the basis of a plurality of ink droplets in this manner, the average of the detected values of a plurality of ink droplets is used as a detection result. It is therefore difficult to detect a fluctuation or variation in each ink droplet.